Re: How can I predict which regions xdisp will present me for font-locking?

[Eli Zaretskii <eliz@gnu.org>]

> Date: Sun, 11 Mar 2012 20:59:03 +0000
> From: Alan Mackenzie <acm@muc.de>
> 
> In a file.hpp, inside a 3355 line macro (I kid you not), I type a single
> character.  xdisp responds by presenting jit-lock, successively, with
> four areas to fontify.  (This was determined by
> trace-function-background on `font-lock-fontify-region').
> 
> None of these areas starts/stops at the top/bottom of the window.  It is the
> third area which encloses the changed line.  The fourth area finishes
> below the window's last line.
> 
> I can't see how this wierd sequence of fontification areas can be
> being calculated by CC Mode.
> 
> This seemingly superfluous fontification seems to be causing a
> performance degradation.
> 
> How can this series of fontification regions be explained?

I think what happens is this:

  . every change in the buffer triggers a call to
    font-lock-extend-jit-lock-region-after-change (via
    jit-lock-after-change-extend-region-functions)

  . font-lock-extend-jit-lock-region-after-change extends the region
    of the change according to the font-lock properties of the major
    mode, then marks that region unfontified by putting on it a
    `fontified' text property with the value nil

  . the next redisplay cycle calls jit-lock-function (via
    fontification-functions) at the beginning of the region whose
    `fontified' property is nil, and refontifies the region from there
    in chunks of 500 characters (but every chunk is extended so that
    its start is at the beginning of a line and its end is at the end
    of a line)

Does this explain what you see?